This invention relates to galvanic cells having rolled or coiled electrode assemblies, i.e., so-called jelly roll electrode assemblies.
Such cells normally have at least one electrode of a layer of electrochemically active material supported on a carrier layer to form such electrode, e.g., the cathode. The carrier layer is a conductive support material, normally metal. Although some manufacturers employ carriers of expanded metal, sheet metal, or the like, the assignee herein prefers to employ a thin delicate metallic foil as the carrier so that, among other things, the thin foil carrier occupies far less space in the cell, allowing a correspondingly greater quantity of active electrode material. Although a grid carrier can be used, such a carrier is thicker, heavier and more expensive than a thin metal foil carrier and thus less volumetrically efficient in the cell. Moreover, it is desired to have the foil project axially further than the separator in the cell to be exposed as a helix of foil, so that a contact member can engage the projecting foil to make electrical contact and serve as the terminal. The use of foil, however, especially in this projecting fashion, presents real difficulties during assembly.
Specifically, battery assembly employing wound electrodes is complicated by the protruding, straight contact edges where the bare foil is exposed at the end of the roll. An insulator ring may be employed at the top of the cell to further insure electrical isolation of the exposed edges of the carrier from the cell container. The container serves as the anode terminal. Any contact at all of the foil to the container shorts the cell. Extreme difficulty has been encountered in attempting to insert the insulator ring after the rolled electrode assembly has been placed in the cell container. Therefore, this ring would normally have to be placed on the wound electrode assembly prior to its placement into the container. This is difficult. The thin carrier edges tend to buckle under pressure in an unpredictable manner. A portion of the foil can readily crumple causing it to contact the container and result in a shorted cell. This also limits allowable contact pressure of a contact spring or other contact member. Cell interior space is limited so that space for an insulator ring and other additional cell parts must be kept to a minimum. Therefore, forcing all of the elements together as is necessary increases the potential for shorting.
Previous methods of making electrical contact to exposed edges of electrode carriers in a rolled cell assembly have included the use of special slotted cross members to grip the carrier edges as disclosed in U.S. Pat. No. 3,732,124. Alternatively, the edges of a sheet metal electrode carrier have been slit and each section folded over to partially overlap the adjacent folded section as in U.S. Pat. No. 3,761,314 (see especially FIG. 3). The first technique requires exact fitting of the slotted member prior to cell sealing, a tedious and time-consuming operation. The second technique requires careful multiple slitting of the carrier edge, with the same disadvantage. Further, these folded over portions can block substantial areas of liquid access to the cell interior during electrolyte filling, thus increasing the time required to assure complete filling, and possibly blocking some electrode portions from any electrolyte contact.
The only foil backed nonaqueous solid cathode cell known to be commercially sold has been a Li/MoS.sub.2 cell which employs an electrode tab welded to the pin of a glass-to-metal seal, the pin thereby serving as the terminal for that electrode. The foil edges of this cell are not exposed but are covered by a wider separator strip. This is a different type of construction.
Another manufacturer employs an expanded metal carrier, with a pressure contact to one electrode in its Li/MnO.sub.2 cell, using an electrode tab on the other electrode rather than contact to exposed carrier edges. Still another manufacturer employs pressure contact to exposed electrode carrier edges at the top of its Li/MnO.sub.2 cell, but using an expanded metal grid carrier which can be radially crimped toward the center without crumpling the carrier or causing shorting thereby. This cell has a plastic insulator cone at the top.